5 Answers
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In fact, this is a specific technique known as a "coffee toddy" which is a good way to take liquid coffee camping with you if you don't want to take brewing equipment. Some people even prefer coffee made this way to other methods of brewing.

Yes I would imagine that is would be possible to heat cold-brewed coffee without losing flavour (excuse my Canadian spelling of "flavor"). It's not the heating of the coffee that makes it lose it's flavour, it's the staleness of it. When coffee is brewed it is full of flavourful oils, but once those oils become stale the coffee loses its taste.

So, as long as cold-brewed coffee is fresh, you won't lose any flavour by heating it. Of course, once it is stale it WILL lose its flavour.

Edit: It's my understanding that cold-brew coffee takes a much longer time to make than heated coffee, and probably takes longer to become stale, but I'm not familiar with the mechanics of cold-brew or how long it would take to become stale.

@keflavich Forgot to mention that oxidation is a result of contact with air. However I'm not sure what would cause an increase in acidity, or whether that's really a cause of staleness. Even if coffee is stored away in a container and not exposed to air it will still oxidize by the oxygen and other compounds dissolved in the water (albeit a little bit slower than if exposed to air).
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BizorkeDec 19 '11 at 21:59

I tried this today. Yesterday I soaked a tsp of illy in water for 24 hrs. It came up quite good. Not very bitter. I filtered it and heated it on very low heat, (a setting of 3 out of 6 on my hot plate) until it started to give off a little steam. Now it felt quite bitter on tasting.

I always make my hot coffee in a moka on this temprature and its doesnt burn, So I dont think I burnt my cold brew

My experience of heating coffee in a microwave is very negative, although I cannot explain the mechanism for it turning out so awful. It may be the effect of the hot coffee cooling down, rather than the effect of the reheating.

We've all experienced coffee that's been sat on the hotplate for too long; bitter and acrid. You risk this "burning" effect if you heat cold coffee on a hotplate.

Cold press coffee is generally brewed very strong indeed; about twice as strong as espresso. Hence you can add boiling water at a ratio of, say, 3:1 and have an acceptably warm cup of coffee - and this is what people usually do.

If you want to microwave for extra heat, just microwave the full-strength cold-brew for 10 seconds or so, before adding water from the kettle.